18. Find the value of r, if P(5,r) = 2.P(6,r-1) ?
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Answer:
r - 4
Explanation:
(5−r)!
5!
=
{6−(r−1)}!
6!
⇒
(5−r)!
5!
=
(7−r)!
6×5!
⇒
(5−r)!
1
=
(7−r)(6−r)(5−r)!
6
⇒ 1=
(7−r)(6−r)
6
⇒ (7−r)(6−r)=3×2
⇒7−r=3⇒r=4
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